1. Field of the Invention
This invention relates generally to steam turbines and, more particularly, to a method of salvaging stationary blades of a steam turbine during replacement of a cylinder.
2. Description of the Related Art
A steam turbine power generating system usually includes a combination of low pressure, intermediate pressure, and/or high pressure steam turbines which are coupled together to provide a single power output. Each steam turbine includes a rotor having a plurality of rotating blades mounted thereon in rows with the blades of a given row being identical to each other. The rotating blades of a row extend radially outwardly from an outer surface of the rotor, with the rows being spaced apart in pairs on opposite sides of a transverse symmetry plane of the rotor. Since steam passes from the center of the rotor outwardly in opposite directions, the rotating blades of a row pair are substantially the same but oriented in opposite directions. The rotating blades of one row pair differ in shape from those of the other row pairs; most noticeably, the rotating blades of each row pair, or stage, become progressively longer as the distance along the rotor away from the transverse symmetry plane increases.
Each rotating blade, regardless of row, has an airfoil portion extending radially outwardly from the rotor and a base portion for mounting the blade to the rotor. The base portion includes a root which is fitted into an annular mounting groove provided for each row, and a platform at the proximal end of the airfoil portion. The airfoil portion has a tip at the distal end and has a twist profile from the proximal end to the distal end.
A stationary cylinder is coaxially supported around the rotor and has a plurality of stationary blades mounted on an inner surface thereof. The stationary blades are arranged in rows which, when the cylinder is assembled with the rotor, alternate with rows of rotating blades. The stationary blade rows are disposed axially along an inner surface of the cylinder in spaced apart pairs on opposite sides of a transverse symmetry plane of the cylinder which is coplanar with the transverse symmetry plane of the rotor. Like the rotating blades, the stationary blades of a row pair are substantially identical but oriented in opposite directions. The stationary blades of one row pair are shaped differently than those of the other rows, although all stationary blades have an airfoil portion and a base portion including a root and platform. The platforms of adjacent stationary blades of a row abut each other and the tips of the airfoil portions are shrouded together by a series of shrouds, each one usually interconnecting four or five stationary blades. The shrouded tips of the stationary blades are in contact with steam seals provided on the rotor between adjacent rotating blades. Similarly, tips of the rotating blades are in contact with steam seals provided on the cylinder between adjacent stationary blades.
The roots of the stationary blades of a row are mounted in an annular mounting groove provided in the cylinder. A typical annular mounting groove for a row of stationary blades has opposite side walls and a bottom wall. An annular recess is provided at the bottom wall, which extends axially into one of the side walls of the annular mounting groove. The root of each stationary blade is provided with a side notch which, when the root is fitted into the groove, aligns with the annular recess. The side notch and the annular recess together define a space which is common to both the cylinder and the root. When the space is filled with caulking material, the cylinder and root become keyed together. The caulking material is a soft metal which is applied with a caulking tool, such that as the stationary blades are fitted into the mounting grooves, a piece of caulking material is inserted into the space from an end of the root and then deformed to fill the space, thereby forming a key between the root and the cylinder. After caulking, the blades are shrouded together in groups.
The cylinder of every steam turbine is subjected to thermally induced stress which, over time, will degrade the condition of the cylinder to the point of requiring replacement. The presently employed maintenance procedure for replacing the cylinder involves separating the cylinder from the rotor, discarding the old cylinder along with the stationary blades connected thereto, and installing a new cylinder having new stationary blades. In order to facilitate assembly and disassembly, the cylinder is formed in two axially split halves which are bolted together. The maintenance procedure of replacing the cylinder, while not frequently required, is costly. Since one cylinder may have over 1,200 stationary blades, the cost of the stationary blades represents a substantial portion of the overall cost of a replacement cylinder. Usually when the cylinder needs replacement, the stationary blades are still in good condition.